1. Field of the Invention
This invention relates generally to lithographic processing. More particularly, this invention relates to a maskless optical writer for direct writing high resolution patterns to substrates such as integrated circuit wafers.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. During lithography, a wafer is disposed on a wafer stage and held in place by a chuck. The chuck is typically a vacuum or electrostatic chuck capable of securely holding the wafer in place. The wafer is exposed to an image projected onto its surface by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the relevant art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove, dope, or otherwise affect exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and the reticle or light valve that defines the pattern simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of a pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized.
Reticles (also known as masks or photomasks) are used to block photoresist exposure in selected areas, defining the pattern to be exposed. Reticles, and the use of reticles, can be expensive, especially for small wafer runs.
An alternative to using reticles is to use a maskless light valve called a spatial light modulator (SLM), such as a grating light valve (GLV) or a digital micromirror device (DMD) (also known as a digital micromirror array or a tilt-mirror array). A DMD is an array of a multitude of tiny mirrors, each mirror representing one pixel of a pattern. Each micromirror can be individually programmed to be turned on or off, thereby allowing the micromirror array to be programmed to represent a desired pattern. When an individual micromirror is turned on, an illumination is reflected by that mirror toward exposure optics and ultimately to a photoresist or substrate (e.g., a wafer). When an individual micromirror is turned off, the illumination is not reflected toward the exposure optics and therefore is not then reflected toward the photoresist or substrate. In this way, the DMD becomes a maskless light valve.
One disadvantage of using a DMD is that micromirrors generally can only be on or off. In other words, a DMD does not easily allow greyscaling. In order to change greyscaling using a DMD, the micromirror needs to be precisely tilted to an exact angle. However, the customized tilting of the micromirror can have negative effects. For example, telecentricity may change due the image plane no longer being orthogonal to the optics as a result of a mirror tilt. In addition, with a DMD, one does not have control of the phase of the light for individual pixels. Therefore, a DMD cannot easily be used as a phase shift mask.
What is needed is a maskless optical writer for direct writing patterns to substrates, without the disadvantages associated with reticle systems and known light valve systems described above.